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Post Info TOPIC: AA Doradus
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RE: AA Doradus
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Title: Recent Investigations on AA Doradus
Authors: Thomas Rauch (Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Tuebingen, Germany)

AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary and an unseen low-mass secondary, believed to be a brown dwarf. Eleven years ago, a NLTE spectral analysis of the primary showed a discrepancy with the surface gravity that was derived by radial-velocity and light-curve analysis that could not be explained.
Since then, emission lines of the secondary were identified in optical spectra and its orbital-velocity amplitude was measured. Thus, the masses of both components are known, however, within relatively large error ranges. The secondary's mass was found to be around the stellar hydrogen-burning mass limit and, thus, it may be a brown dwarf or a late M-type dwarf. In addition, a precise determination of the primary's rotational velocity showed recently, that it rotates at about 65% of bound rotation - much slower than previously assumed.
A new spectral analysis by means of metal-line blanketed, state-of-the-art, non-LTE model atmospheres solves the so-called gravity problem in AA Dor - our result for the surface gravity is, within the error limits, in agreement with the value from light-curve analysis.
We present details of our recent investigations on AA Dor.

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Title: AA Doradus and its Cool Companion - The Influence of Enhanced Metal Line Blanketing and the Reflection Effect
Authors: Sebastian Müller, Stephan Geier, Ulrich Heber

AA Dor is one of only twelve so called HW-Vir systems, which are perfectly suited to determine fundamental properties like radii and masses of the components. These systems are hot subdwarf binaries which additionally show eclipses in their light curves. So far AA Dor has been studied in many investigations, however, a controversy about the nature of the unseen companion still persists. Since the influence of substellar companions on the evolution of hot subdwarfs is still unclear, the question whether the companion is a brown dwarf or a low mass main sequence star is a crucial one. To reveal the companions nature, we re-analysed high resolution spectra using metal enhanced LTE model atmospheres. Since the optical spectra are polluted by reflected light from the secondary component, we used only spectra taken during epochs when the secondary was eclipsed. Besides the atmospheric parameters effective temperature T_eff, surface gravity log(g), the helium abundance log(y) and the projected rotational velocity v_rot*sin(i) we determined the companion's radial velocity amplitude to evaluate the masses of the system. For the first time a self-consistent solution is achieved, i.e., previous discrepancy of the atmospheric parameters has been removed. Our quantitative spectral analysis leads to T_eff=37300±500 K and log(g)=5.46±0.05. The resulting masses are M_sdB=0.51±0.13 solar masses for the primary and M_comp=0.085±0.023 solar masses for the secondary, which are in agreement with the canonical mass of an hot subdwarf star and a low-mass main-sequence star. However, a brown dwarf nature of the can not be excluded.

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